Inferno was the first story in which producerBarry Letts had any say on development and commissioning. DirectorDouglas Camfield fell ill after completing parts one and two. This left Letts to direct the other five parts.

Several possible stories were considered for the season finale. The Mists Of Madness by Brian Wright and The Shadow People by Charlotte and Dennis Plimmer were two. However, Wright was unavailable and the Plimmers abandoned the project after a pay dispute. This left the way open for The Mo-Hole Project, as it was then titled, by Don Houghton. The serial went through several name changes before becoming Inferno.

Houghton had based the drilling part of the story on real events. He had contacted the American Embassy to enquire about Project Mohole, a real world attempt to drill through the Earth's crust. Information on the project and why it was abandoned was not disclosed as such information was top secret. It was this secrecy that Houghton developed upon for his story.

Letts was worried when the first script draft arrived as he did not think there was enough in it to fill a seven-parter. Terrance Dicks decided upon the idea of a parallel universe and Houghton was happy to oblige. Further additions to the script were the Primords and Venusian aikido.

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UNIT is providing security cover at an experimental drilling project designed to penetrate the Earth's crust and release a previously untapped source of energy. Soon however the drill head starts to leak an oily green liquid that transforms those who touch it into vicious primeval creatures with a craving for heat.

The Doctor is accidentally transported by the partially repaired TARDIS control console into a parallel universe where the drilling project is at a more advanced stage. Thwarted by his friends' ruthless alter egos, he works to save both universes.

The Inferno Project is the nickname given to a project to penetrate the Earth's crust to obtain pockets of Stahlman's gas at its core, which is theorised to be able to provide endless, massive amounts of energy. Professor Stahlman himself is ill-tempered and obsessive about any interference with the progress of his work. Sir Keith Gold, the project's executive director, is concerned about this and tells Petra Williams, Stahlman's assistant, that he has called in Greg Sutton, an oil drilling expert, to consult on safety issues.

UNIT is overseeing security at the project, and the Third Doctor is at the facility for his own reasons. He is using the output of the project's nuclear reactor to power experiments in the control room, which has been made portable, in an attempt to end his exile on 20th century Earth imposed upon him by the Time Lords.

The project, however, has its own problems. Slocum, a worker repairing one of the drill pipes, encounters a toxic green slime seeping out of one of the output pipes. Upon contact, the substance rapidly mutates him into a feral creature, which goes berserk and kills one of the other workers.

While the Brigadier and Sergeant Benton investigate, the disfigured Slocum enters the reactor control room and attacks Bromley, the technician there, then pushes the reactor power to maximum.

At this time the Doctor is using the TARDIS console and the energy rush shifts the console and the Doctor into a dimensional void from which he is barely saved when Liz manages to cut off the power flow. Alarms go off as the drill is in danger of shutting down due to the overload. When the Doctor reaches the reactor control and starts to lower the power output, Slocum bursts through a door, into the room, ready to kill....

The Doctor tries to calm Slocum down while he screeches horribly but a UNIT soldier, Wyatt, tries to shoot Slocum and is attacked. Slocum eventually collapses, as does Wyatt, and the former's body is so hot that it scorches the wall where he falls.

The Doctor voices his concern that Slocum's mutated form seems to be filled with some strange power. He has also heard the screeching before — during the 1883 explosion of Krakatoa. Benton informs the Brigadier that Bromley and Wyatt have gone missing, before their medical checkups. The Doctor encounters Wyatt, who is in the process of mutating, on the cooling towers. Wyatt falls to his death, but the Doctor fails to notice the mutated Bromley on the tower as well.

The drill has been bringing up more quantities of the green slime, but Stahlman dismisses any connection between that and the incidents at the project. When a jar of the slime starts to bubble over, he quickly grips it, superficially burning his hand. Stahlman places the jar in a box which he orders frozen, and secretly sabotages the project computer, which was predicting disaster, by stamping on the computer's micro-circuit. He also orders that the power be cut off to the Doctor's hut, so that the drilling can be accelerated, with penetration occurring in 49 hours. The Doctor, in the meantime, sends Liz away on a wild goose chase while he hooks up the console again. Noticing the power drain, Stahlman cuts the power to the console just as Liz and the Brigadier rush back to the hut. As they look on, the Doctor, his console, and his car Bessie vanish into thin air...

Liz tells Stahlman he must restore power to the Doctor's hut, but the Professor is indifferent.

The Doctor wakes up apparently still in the hut, but when he drives away in Bessie he is shot at by soldiers, including Benton. Benton and his men follow and kill a Primord. Just as he believes he has shaken off pursuit, he encounters Liz dressed as a soldier. She captures him and takes him for questioning to Brigade Leader Lethbridge-Stewart, a sinister version of the Brigadier with an eye-patch.

None of the people encountered are the people he knows, as the Doctor quickly deduces that he has travelled to a parallel universe. The Inferno project is also ongoing in this universe and is slightly more advanced. The Doctor is presumed to be a spy because of his knowledge on the project.

The Section Leader enters and stops Benton from killing the Doctor. It is only with the Doctor's help that the crisis is averted, but he is interrogated and thrown into a cell regardless.

He tries to convince the parallel versions of his friends that he is from another universe, but they believe he is trying to feign insanity. Here, Stahlman has also been infected with the green slime, and is mutating. The Doctor is interrogated by the Section Leader and Brigade Leader, and he is taken away after he asks Stahlman to take off his glove. It does no good, since he has his hand bandaged. The Doctor is placed in a cell with a sedated Bromley, but the alarm is raised when Bromley awakens fully mutated. Meanwhile, in the Doctor's universe, Sir Keith has decided to go to the minister to try and slow the project down.

Escaping, the Doctor makes his way to the main control room to stop the drilling but is discovered. The Doctor pleads for them to stop, telling them that the screeching is the sound of the planet "screaming out its rage." Stahlman holds him at gunpoint, with the countdown at 1 second before penetration...

As Stahlman holds a gun on the Doctor, penetration is achieved, an explosion is heard, and an earth tremor rocks the installation. The temperature rises rapidly as more green slime oozes out of the cracked pipes. As the Doctor and the parallel Sutton try to contain the explosion, Stahlmann attacks them. They manage to escape, leaving Stahlman behind a heat shield with the bodies of the unconscious workers. Stahlman rubs the slime on their faces, mutating them as well. There are seismic disturbances all over the country, and the Doctor explains that now that the crust has been penetrated, the planet will soon revert back to the gases it sprung from, saying they have "reached the point of no return", and that they will never be able to plug the hole, as nothing can withstand the immense pressures of the mantle.

When one of the Primords attack, the surviving people at the base find that they can be killed by cold from fire extinguishers. The government abandons them to their fate. The Doctor tries to convince the others that he can stop this from happening in his own universe if they will help him to return, and shows them the TARDIS console. He drains all energy from the TARDIS storage unit, and the Brigade Leader asks him to bring them back if they give him the necessary power. Meanwhile, Stahlman has fully converted into a Primord, and, in the original universe, Sir Keith's car has crashed.

The Brigade Leader demands that the Doctor save them, too, but the Doctor says that they do not belong in the other universe. Refusing to accept this, the Brigade Leader orders everyone back to the control room, where Stahlman and his fellow mutants attack, infecting Benton as well. In the office, the Doctor starts to explain his plan, but a Primoid's arm breaks through the glass...

Finally agreeing to help the Doctor, the group fights their way out using fire extinguishers to paralyse the mutants. By this time, the sky has turned red and the heat is overwhelming. The parallel Petra Williams, with Greg Sutton's help, manages to feed power to the TARDIS console. At the last moment, the Brigade Leader snaps and threatens to shoot the Doctor if he doesn't save them, but is gunned down by Section Leader Shaw. A wall of lava sweeps towards the hut and the others watch the end coming towards them.

The Doctor is back but unconscious in a healing coma, and there are only three hours left before penetration zero. When he awakes, the Doctor goes to the main control room and tries to smash the controls. He is unsuccessful and has to be restrained, but manages to tell Liz to put a new circuit into the computer that Stahlman had sabotaged. Liz does so, and the computer advises drilling be stopped at once. In the meantime, Stahlman orders everyone out of the drill head area, then when they are gone he picks up a handful of slime and rubs it into his face, causing himself to completely mutate. The Doctor escapes from the sickbay and returns to the control room, dealing with the Bromley mutant on the way. The Stahlman mutant emerges and has to be subdued with fire extinguishers. With seconds to go, the drill is shut down. The Doctor recommends to Sir Keith that arrangements be made for the shaft filled in.

Later, Sir Keith, who suffered only minor injuries in the crash, informs the Doctor that the project is being abandoned and everyone is leaving. The Doctor announces that he, too, is leaving. The Brigadier and Liz protest, and the Doctor sharply tells the Brigadier that he reminds him of his fascist counterpart. The Doctor activates the console and vanishes; on this occasion, Bessie is left behind. A few minutes later, the Doctor appears at the door of the hut, with mud on his clothes – having only made it as far as a nearby garbage dump! Suitably chastened, he asks the Brigadier to help him retrieve the console, which has landed in a somewhat inaccessible position, much to Liz's amusement.

This story had the working titles of The Mo-Hole Project, Operation: Mole-Bore, The Mole-Bore and Project Inferno.

The primordial mutant creatures are named 'Primords' in the closing credits of the episodes in which they appear and in publicity material, but are unnamed in the story's dialogue.

In the first scene featuring the Brigade Leader, Nicholas Courtney turned round in his swivel chair to find himself seeing all the other cast and crew wearing eye patches, but he continued the entire scene with a straight face.

The opening credits for this serial were unusual in that after the initial titles, the name and part number of the serial were superimposed on footage of a lava flow, with no music.

The Radio Times programme listing for episode one was accompanied by a black and white rehearsal shot of the Doctor standing at the TARDIS console, with the accompanying caption "Knock, knock. Who's there? 5.15", while that for episode seven was accompanied by a black and white photograph of the mutated Director Stahlman, with the accompanying caption "Meeting the Primord. Dr. Who: 5.25".

In spite of Douglas Camfield receiving sole credit as director, the second studio recording block and studio blocks 3 & 4 were directed by producer Barry Letts after Camfield had a minor heart attack on 27 April 1970. Letts later stated that Camfield's preparations for episode one's studio recording were so meticulous that he just followed Camfield's existing camera script. For the other episodes, Letts worked to his own plans, which he had to hurriedly construct. In the end, Camfield directed all the location filming and designed the direction executed by Letts on episode one's studio recording. Nevertheless, Camfield remained credited as director, as BBC regulations at the time forbade the same person being credited as producer and director.

Derek Ware did not perform the scene where the mutated RSF Private Wyatt is shot and falls to his death from the top of one of the cooling towers, in case he was injured, as he was also needed for studio recording. His place was taken by Roy Scammell who, curiously, also played the RSF sentry who fires the fatal shot. Ware stated in an interview that Scammell had already signed the contract to do the fall before Ware had been cast as Wyatt.

Episode six has a small damaged section on the tape, which the Doctor Who Restoration Team replaced by painstakingly recolouring the appropriate section of the existing 16mm black & white film telerecordings.

Caroline John enjoyed her role as Section Leader Elizabeth Shaw and says that it was fun playing 'baddie' Liz. She also says she hated doing the scenes when she was playing the 'goodie' version because it was boring compared to being an evil character. She was particularly upset though about the scene in which Shaw shoots Brigade Leader Lethbridge-Stewart, as she was pregnant at the time. As a result, the scene was recorded with the weapon fired from out-of-shot, after which Shaw was shown returning the gun to her holster.

Nicholas Courtney has also said he enjoyed playing the parallel version of the Brigadier. He called it his "favourite" in an interview with Doctor Who Magazine in 2009. The DWM staff remembered this by including it in his tribute issue as that month's "The Fact of Fiction".

At Doctor Who fan events, Courtney frequently recounted an incident which occurred during shooting of the scene with the revelation of the Brigade Leader and his eyepatch. The scene begins with Courtney's back to the camera (and to all the other actors), and then he spins around in his swivel chair to reveal the Brigade Leader's visage and eyepatch. When the scene was shot, Courtney spun around to discover that all the other actors and crew members were also wearing eyepatches. The "eyepatch story" became so closely associated with Courtney that Steven Moffat wrote scenes in which "everybody was wearing an eyepatch" into the 2011 episode The Wedding of River Song as a tribute to Courtney, who had died earlier that year. (DWMSE 31)

During the scenes set on the parallel Earth, images (supposedly) of the UK's dictatorial leader are seen on posters. The image used is that of visual effects designer Jack Kine, in homage to the 1954 BBC adaptation of George Orwell's Nineteen Eighty-Four where the face of Big Brother was Head of Television Design Roy Oxley (Kine had worked on the visual effects for that production).

The scenes on top of the Nuclear Reactor in episode two were not easy for Jon Pertwee, as he was afraid of heights. It took him fifteen minutes just to summon up the confidence to film the scene.

In episode three, during the chase scene on the parallel Earth, Bessie's bumper gashed open stuntman Alan Chuntz's leg as it passed. The gash was so severe that he had to go to A&E. Barry Letts stated on the DVD commentary that "it was the worst wound [he]'d ever seen" and after Pertwee saw the wound and learned he caused it, he became sick and almost couldn't carry on filming until a few hours later. Jon Pertwee even went as far as buying Chuntz's wife a gift to apologise for the accident.

The parallel universe aspect of the story was added to the scripts at the production team's suggestion to ensure that there was sufficient material to fill seven episodes. (Though Terrance Dicks has claimed this in several documentaries, the parallel universe aspect was present as far back as Don Houghton's original story outline. It actually appears to have been the Primords who were added at the production team's suggestion)

This was Caroline John's last story as Liz as she was pregnant and could not return for the following season. (Although it is true that the actress was pregnant, Barry Letts was unaware of this when he decided against renewing her contract.)

The Doctor's remark that he had heard the screeching sound the Primords made before at Krakatoa implies that he had encountered (or at least heard) the creatures before.

A sketch that washed up on the shores of Sumatra following the Krakatoa eruption was later shown to Rose Tyler, depicting the Ninth Doctor in front of a volcano. (He may well have been in a different area near the vicinity of Krakatoa than his previous self.) (TV: Rose)

In February1970, the journalist James Stevens dismissed Greg Sutton's outlandish claim that a green slime from the centre of the Earth transformed scientists into wolf monsters during the Inferno Project. He described Sutton's story as sounding like the plot of a "science fiction potboiler." (PROSE: Who Killed Kennedy)